A fuel cell stack is a power generation system which oxidizes a fuel through an electrochemical process to thereby directly convert energy released as a result of the oxidation reaction into electric energy. The fuel cell stack has a membrane-electrode assembly in which a polymer electrolyte membrane, which selectively transports hydrogen ions, is sandwiched by a pair of electrodes made of porous materials. Each of the pair of electrodes includes: a catalyst layer that contains, as a main ingredient, carbon powder supporting a platinum-based metal catalyst, and that contacts with the polymer electrolyte membrane; and a gas diffusion layer formed on a surface of the catalyst layer, the gas diffusion layer having both air permeability and electronic conductivity.
In fuel cell systems of this type, if an operation of the fuel cell continues within an operation zone where a cell voltage is an oxidation voltage (about 0.7-1.0 V), a platinum catalyst effective area may be reduced due to the formation of an oxide film on a platinum catalyst surface of a catalyst layer, resulting in a degradation of the performance of the catalyst layer and thus a degradation of the power generation performance. Under such circumstances, Patent Document 1 describes processing for restoring power generation performance by, upon detection of the state in which a fuel cell continues to operate within an oxidation zone where the platinum catalyst is oxidized, decreasing a cathode potential to a reduction voltage (e.g., 0.6 V or lower) and thereby removing the oxide film from the platinum catalyst surface (hereinafter referred to as “refresh processing”).